1. Field of the Invention
This invention relates to actuators for controlling valves to air turbine starters and more particularly to a microvolume actuator that prevents rapid opening of the air turbine starter valve when partially frozen closed.
2. Description of the Related Art
Air turbine type starter motors operate with the energy of a compressed gas such as air and are often used for starting a turbine engine, such as that used on aircraft. The compressed air for the air turbine starter is controlled by a starter valve, such as a pressure regulating and shut-off butterfly valve, or a shut-off valve.
A source of relatively clean dry air is required to power the air turbine starter. The most common source of air for this purpose are an auxiliary power unit, bleed air from the compressor stage of another operating gas turbine engine, or a gas turbine ground cart. Upon actuation of the engine start switch, the starter valve is energized and opens at a controlled rate to permit air to flow to the air turbine starter. The air turbine starter valve output air flow engages the air turbine starter motor, which converts the energy in the air to torque. This torque is applied to the engine gearbox which is then accelerated to a predetermined cut off speed whereupon the engine can accelerate to idle. The start cycle may be terminated manually by the pilot opening the start switch or automatically by a speed sensitive switch built into the starter or by a main engine speed signal to a fully automated digital engine controller (FADEC). When the start cycle is terminated, the starter valve is closed cutting off the energy to the air turbine starter. When starting air is cut off, the air turbine starter automatically disengages from the engine accessory drive shaft and comes to a stop.
The starter valve controls the output torque of the air turbine starter by means of a controlled opening rate of the valve, a controlled closing rate, and/or a pressure regulating system which delivers substantially constant pressure to the starter regardless of the upstream air pressure. These functions in a conventional starter control valve may be implemented by mechanical-pneumatic control devices such as orifices, needle valves, springs and diaphragms. While such devices are generally acceptable, these devices are complex in design and manufacture, may be difficult to adjust, and may be sensitive to environmental changes and may have poor repeatability under certain circumstances.
The starter control valve controls the pressure of the starter air that is initially supplied to the air turbine starter to prevent destructive shock to the mechanism. As the starter responds, the rate of increase in air (fluid) pressure is typically progressive to effect a smooth, rapid acceleration of the starter""s turbine mechanism. In addition, the control valve may serve to regulate air pressure.
While a control valve of this type is generally acceptable, it is difficult for the valve to simultaneously regulate pressure, limit pressure rise rate, and control the speed of the air turbine starter. It is also difficult for the valve to meet strict performance requirements over a wide range of environmental conditions.
When the valve is opened, the relatively large air volume present in the actuator that controls the valve increases in pressure and becomes a reservoir of potential energy. This pressure is usually relatively small to prevent damage to the engine being started. However, when ice is in the start control valve, the valve may initially stick until the actuator develops enough torque to break the ice and open the valve. When this occurs, the pressure behind the valve may force the air into the engine in a generally uncontrolled manner. This initial high pressure spike can damage the air turbine starter, as well as the main engine gearbox.
As shown in FIG. 1, an air turbine starter valve actuator 100 is shown connected to a butterfly plate 202 by a butterfly shaft 102. Pressurized air 206 enters into the duct 204 but is held back by the closed butterfly plate 202. A probe 110 feeds the air flow into the actuator 100. A regulator orifice 112 controls volume and pressure flow into the actuator 100.
To close the plate 202, inlet pressure is ported through the butterfly shaft actuator in-bleed orifice 110 and routed to an inner chamber 116 through the regulator orifice 112. With the solenoid valve 120 de-energized as shown, a larger diameter chamber 124 is pressurized through a transmission orifice 126 so that the larger diameter chamber 124 is generally at the same pressure as the inner chamber 116. A second smaller diameter chamber 130 is continually vented to ambient by an associated vent 132. The resulting pressure differential across the diaphragm 144 sealing the smaller diameter chamber 116 produces an actuator force that assists the torsion spring 142 to close the butterfly plate 202 and to keep it closed.
The transmission orifice 126 is sized to control the rate of pressure change on the larger diameter chamber 124. This produces a controlled time for the closing of the valve.
With the solenoid de-energized as shown in FIG. 1, the inlet pressure is routed simultaneously to the inner chamber 116 and the larger diameter chamber 124 through the regulator orifice 112 and the transmission orifice 126, respectively. The matching of the regulator orifice 112 and the transmission orifice 126 to the volume flow time requirements of the inner chamber 116 and the larger diameter chamber 124 prevents self-opening of the butterfly plate 202 during rapid inlet pressure rate rises.
The actuator 100 opens when the solenoid 120 is energized. The valve ball 150 seats itself in the valve seat 152 generally approximate to the transmission orifice 126. Actuator supply pressure is then vented from the larger diameter chamber 124 to ambient through the valve vent 154. Due to the effective area of the larger diaphragm 140 relative that to the smaller diaphragm 144, the resulting actuator force will overcome the closing torsion spring force to open the butterfly plate 202 and keep it open. The valve vent 154 is adjustable and appropriately sized to control the rate of pressure decay in the larger perimeter chamber 124 to produce a controlled rate of downstream pressure rise during the opening of the butterfly plate 202.
As is common with some valves, the butterfly plate 202 may be opened manually by inserting a square drive tool in the end of the butterfly shaft and rotating the shaft to open the butterfly plate 202. Normal operation is reestablished by rotating the tool to the closed position.
Valves such as the one shown in FIG. 1 generally serve to open, close, and control the connected butterfly plate 202 so long as conditions are not severe. However, should the butterfly plate 202 become obstructed, the energy stored in the chambers of the actuator 100 may over-power the obstruction and the butterfly plate 202, causing the butterfly plate 202 to open too quickly and without a gentle transition from unpressurized air flow to pressurized air flow. Such pressure transitions, or transients, may damage the associated air turbine starter (ATS) and engine gearbox. Damage to the ATS may shorten its useful life and prevent its full and proper operation. In particular, once the ice fails, the butterfly valve is free to open and may do so by snapping open and quickly transmitting a pressure gradient on the order of 2000 psi/second to the air turbine starter.
Damage to an ATS can be especially inconvenient, because it may prevent the starting of an engine on the ground and delay the flight for the replacement of the turbine starter. Additionally, in those rare instances where an in-flight air turbine starting is needed, a damaged air turbine starter can impact the proper operation of the starting procedure affecting aircraft safety. As the starting of the gas turbine engine associated with the air turbine starter is of significant importance, the integrity and operation of the air turbine starter is of similar importance. Consequently, an ATS valve is needed that will prevent damage caused by pressure transients due to icing or other obstructions is desired. The present invention satisfies this need.
The present invention prevents damage to air turbine starters and related gas turbine gearboxes by preventing the generation of sharp air pressure transients from ice-obstructed or otherwise obstructed valves.
Most actuators use a relatively large volume of pressurized air to actuate the coupled butterfly valve. While such actuators do work, they also store a significant amount of stored energy in the form of pressurized air. If the associated valve is temporarily obstructed as by ice, air pressure builds until enough force is brought to bear on the obstruction until it fails. Once the failure occurs and the valve is able to pivot to its open position, it may do so violently or sharply under the pent-up force present in the actuator.
The present invention allows both the generation of sufficient force to break obstructive ice or the like while also simultaneously allowing for immediate dissipation of that force once the valve is free to open. By using a small (or micro) volume, the present invention uses air pressure to generate valve-opening forces but avoids the detrimental side effects of larger volume actuators.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment(s), taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.